GEOLOGY OF THE KILDEER MOUNTAINS 269 
with the fact that a series of remains of Oligocene deposits have been observed 
to extend from Dickinson to the Black Hills, suggests the probability that a 
river formerly flowed from the Black Hills northeastward through this region. 
If this be true, there should be coarse sediments as the mountains are 
approached, which is probably the case. Another thing which tends to con- 
firm the idea that these are river valley deposits is the fact that, scattered 
over the plains, there are buttes apparently as high as White Butte, but which 
are not capped by later Tertiary beds.t 
However, the apparent anomalous deposition on White Butte is 
explained by Leonard in another way: 
_The beds of the White River group are wanting on Black Butte, although 
occurring at a considerably lower level only three miles to the west. In White 
Butte they are, however, found resting directly upon the thick upper sandstone 
of the Fort Union, which outcrops at several points near the base of the western 
slope of the western ridge, and also at its northern end. This sandstone here 
dips strongly to the east, so that within a distance of three miles its dip carries 
it from the top of Black Butte to the bottom of the ridge on the opposite side 
of the valley, where it is over 200 feet lower.? 
It is believed that the beds on Sentinel Butte are lake deposits, and 
that some at least of the beds on White Butte are river deposits, 
especially the pebbly conglomerates. 
At the Killdeer Mountains the greater part of the deposit is 
distinctly lacustrine, but some of the layers are wind worked. At 
the southwestern corner, however, on the south side of South Pass 
in the SW. { Sec. 31, there are various remnants that suggest a river 
inlet. On the south face of this isolated hill there is an outcrop of 
coarse, yellow sandstone which is interbedded irregularly with 
bright-red, hematitic sandstone. It shows interrupted deposition 
by cross-bedding and lenses. This coarse sandstone is about 15 
feet thick, covered by finer, more loosely cemented, pale, yellow- 
gray sandstone. It is a small outcrop, extending about 200 feet 
and abruptly ceasing. Above the sandstone there are water-worn 
pebbles, as large as three inches in length, scattered over the surface. 
These were traced back into heavy limestone masses, which show 
pebbles widely scattered on weathered surfaces. In a few places 
there is a thin ledge of very coarse sandstone carrying small pebbles; 
t Earl Douglass, op. cit., p. 288. 
2 A. G. Leonard, Geol Surv. North Dakota, 5th Bien. Rpt. (1908), pp. 65-66. 
